Well, if there would be some of use... 
root@debian:/home/ipa# efibootmgr -b 000F -B -v
Could not delete variable: Invalid argument

strace

openat(AT_FDCWD, "/sys/firmware/efi/efivars/Boot000F-8be4df61-93ca-11d2-aa0d-00e098032b8c", O_RDONLY) = 3
ioctl(3, FS_IOC_GETFLAGS, [0])          = 0
close(3)                                = 0
unlinkat(AT_FDCWD, "/sys/firmware/efi/efivars/Boot000F-8be4df61-93ca-11d2-aa0d-00e098032b8c", 0) = -1 EINVAL (Invalid argument)
write(2, "Could not delete variable", 25Could not delete variable) = 25
write(2, ": Invalid argument\n", 19: Invalid argument
)    = 19
exit_group(15)                          = ?
+++ exited with 15 +++